Magnetoplumbite-type (M-type), sintered ferrite magnets are used in various applications including rotating machines such as motors, generators, etc. To provide smaller and lighter rotating machines for automobiles, and higher-efficiency rotating machines for electric apparatuses, sintered ferrite magnets having higher magnetic properties are recently demanded. Particularly demanded to provide smaller and lighter rotating machines for automobiles are sintered ferrite magnets having high Br, as well as high HcJ that enables the magnets to avoid losing magnetism by a demagnetization field generated when made thinner.
M-type, sintered ferrite magnets such as Sr ferrite, Ba ferrite, etc. are produced by the steps of (a) mixing iron oxide with a carbonate of Sr, Ba, etc., (b) calcining the resultant mixture to cause fertilization to obtain calcined clinker, (c) subjecting the calcined clinker to coarse pulverization and wet, fine pulverization to an average particle size of about 0.5 μm together with SiO2, SrCO3, CaCO3, etc. for controlling sintering, Al2O3 or Cr2O3 (if necessary) for controlling HcJ, and water, (d) molding slurry of fine ferrite particles in a magnetic field and drying them, and (e) sintering the resultant molding. Sintered ferrite magnets are machined to shapes for applications.
In the above production method, if fine particles in the slurry after wet, fine pulverization have a small average particle size, the removal of water from a green body during molding in a magnetic field takes an extremely long period of time, resulting in drastically reduced molding efficiency (number of moldings obtained per a unit time), and thus more expensive sintered ferrite magnets. This problem is particularly severe at an average particle size of less than 0.7 μm. Although a larger average particle size improves the molding efficiency, the resultant sintered ferrite magnets have low magnetic properties. In dry molding, too, fine pulverization lowers the molding efficiency, needing magnetic powder having a large average particle size to some extent.
Japanese Patent 3181559 discloses a sintered ferrite magnet comprising hexagonal crystal ferrite as a main phase, and having a composition represented by the general formula of Ca1-xRX(Fe12-yMy)zO19, wherein R is at least one element selected from the group consisting of rare earth elements including Y and Bi, La being indispensable, M is Co and/or Ni, x, y and z meet the conditions of 0.2≦x≦0.8, 0.2≦y≦1.0, and 0.5≦z≦1.2. Japanese Patent 3181559 describes in the paragraph [0018] and Example 6 that the sintered ferrite magnet has about 2% higher saturation magnetization (4πIs) and about 10% higher anisotropic magnetic field (HA) than those of Sr ferrite (Sr M). Sintered ferrite magnets having such high properties are expected to have high potential not achieved by the Sr M. Namely, Br of 4.6 kG (460 mT) or more and about 10% increase of the maximum HcJ are expected. However, Japanese Patent 3181559 shows in FIG. 2 that the magnetic properties of Sample 2 of Example 2 (sintered with 20% of O2) are Br of 4.4 kG (440 mT) and HcJ of 3.93 kOe (313 kA/m), much lower than expected. Thus, there is much room for improvement.
JP 11-97225 A discloses an anisotropic, sintered magnet comprising hexagonal, magnetoplumbite-type ferrite as a main phase, and having a composition of Ba, R, Fe and M represented by the general formula of Ba1-xRx(Fe12-yMy)zO19, wherein R is at least one element selected from the group consisting of rare earth elements (including Y) and Bi, M is Co or (Co+Zn), 0.04≦x≦0.9, 0.3≦y≦0.8, and 0.7≦z≦1.2. Calcined bodies corresponding to the anisotropic sintered magnets shows in Table 1 of JP 11-97225 A have compositions containing particularly a smaller amount of Ca, outside the basic composition range of the present invention. Also, the magnetic properties (Br and HcJ shown in FIG. 1) of the sintered ferrite bodies are not necessarily satisfactory for high-performance demand.
WO 05/027153 A discloses a sintered ferrite magnet having an M-type ferrite structure, and produced from A elements such as Sr or Sr and Ba, an R element that is at least one of rare earth elements including Y and indispensably includes La, Ca, Fe and Co as indispensable elements, by pulverizing, molding and sintering an oxide-type, magnetic material. The oxide-type, magnetic material is represented by the general formula (1) of A1-x-yCaxRyFe2n-zCozO19 (by atomic ratio), and the sintered ferrite magnet is represented by the general formula (2) of A1-x-y+aCax+bRy+cFe2n-zCoz+dO19 (by atomic ratio). In the formulae (1) and (2), x, y, z and n represent the contents of Ca, the R element and Co and a molar ratio, and a, b, c and d represent the amounts of the A elements, Ca, the R element and Co added in the pulverizing step, meeting 0.03≦x≦0.4, 0.1≦y≦0.6, 0≦z≦0.4, 4≦n≦10, x+y<1, 0.03≦x+b≦0.4, 0.1≦y+c≦0.6, 0.1≦z+d≦0.4, 0.50≦(1−x−y+a)/(1−y+a+b)≦0.97, 1.1≦(y+c)/(z+d)≦1.8, 1.0≦(y+c)/x≦20, and 0.1≦x/(z+d)≦1.2. This sintered ferrite magnet is outside the basic composition range of the present invention in that Sr is contained indispensably, and that the content of Sr or (Sr+Ba) is more than the Ca content. Although the sintered ferrite magnet described in WO 05/027153 A has high magnetic properties, it still fails to meet increasingly higher users' demand for performance. Further improvement of the magnetic properties is thus demanded.
WO 06/028185 A discloses an oxide-type, magnetic material comprising hexagonal, M-type magnetoplumbite ferrite as a main phase, which has a composition represented by the formula of (1−x)CaO-(x/2)R2O3-(n−y/2)Fe2O3-yMO, wherein R is at least one element selected from the group consisting of La, Nd and Pr, indispensably including La, M is at least one element selected from the group consisting of Co, Zn, Ni and Mn, indispensably including Co, and x, y, n are molar ratios meeting 0.4≦x≦0.6, 0.2≦y≦0.35, 4≦n≦6, and 1.4≦x/y≦2.5. Because of Ba-free, the oxide-type, magnetic material described in WO 06/028185 A is outside the basic composition range of the present invention, and its magnetic performance is unsatisfactory for recent demand for higher performance.